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Last Updated: Monday, 20 December, 2004, 16:24 GMT
'Eye in the sky' watches over air
By Molly Bentley
in San Francisco

Concept image of Aura (Nasa)
Aura was launched in July
The air around us seems abundant from Earth. But from space, our endless blue sky is only a thin shield around a massive planet.

Our atmosphere is fragile; yet, it is a lifesaver.

Without it, the Earth would be frozen, lifeless, and pummelled by cosmic radiation.

So it is in our interest to protect it. Now a recently launched satellite is beaming back information that may help.

In orbit since July to monitor ozone, climate change and air quality, the US space agency's (Nasa) Aura satellite has already produced the first direct measurements of lower atmospheric - tropospheric - ozone from space, including chemicals that are a precursor to "bad ozone" at ground level, and those that form high levels of ozone over the tropics.

It has also provided new images of the ozone hole over Antarctica.

You emit a molecule of CO2 when you exhale, and in two weeks someone can be breathing that molecule of CO2 in Beijing, when you exhaled it in New York City
Dr Phil DeCola, Aura Program Scientist
With colourful high-resolution images, and a bit of animation, scientists can watch chemical reactions in the atmosphere daily, such as the conversion of safe chlorine to the dangerous form that destroys stratospheric ozone.

It is an unprecedented look at the health of the swirling mix of trace gases that protect life on Earth, and also the chemical reactions that threaten it.

Treaty checks

"Our results are exceeding our wildest expectations," said Phil DeCola, Nasa's Aura Program Scientist, who reported at the Fall Meeting of the American Geophysical Union (AGU) in San Francisco.

Dr DeCola said that the view of the atmosphere from space was essential to understand its composition and chemical dynamics.

City pedestrians cover their mouths in a smog (AFP)
We can do something about the quality of the air we breathe
Current air quality measurements are limited by less sensitive ground-based instruments.

"You'd need millions, perhaps billions of sensors on the surface to get this kind of information," said Dr DeCola.

Scientists say Aura will tell them whether the ozone layer is recovering and whether atmospheric treaties such as the Montreal Protocol - designed to reduce ozone-depleting substances in the atmosphere - are working.

Aura monitored both the good stratospheric ozone that shields us from ultraviolet radiation, and the toxic ozone below, in the air we breathe, explained Aura Project Scientist Mark Schoeberl.

"Ozone will attack your lung tissue and make you really sick," he said. "So we're interested in air pollution, a component of which is ozone. It's a critical issue for urban, mega-city environments."

Car exhausts, chemical solvents and industrial emissions all can lead to bad ozone.

Chemical mystery

The satellite will also monitor the effect of climate change on atmospheric gases and tiny particles or aerosols.

Aerosols add an uncertain element to climate change because they are highly variable in effect - either reflecting or absorbing heat, depending on their type and where they reside in the atmosphere. Their overall contribution to climate change is not clear.

Just as you can tune your radio to classical music or rock, we build these fancy radio receivers and we tune into certain molecules
Joe Waters, Jet Propulsion Laboratory
While the Antarctic ozone hole seems to be on the mend, climate change could create a new hole at the North Pole.

There is still about seven times the natural amount of chlorine in the atmosphere, left over from the chlorofluorocarbons (CFCs) of several decades ago, according to Joe Waters, a principal investigator from the Nasa Jet Propulsion Laboratory (JPL).

It will take 30 years or so to clean out that chlorine. At the same time, a cooling of the stratosphere brought on by climate change, he said, could produce ice, which can convert that chlorine to the dangerous form.

"So one of the things we're watching, particularly in the Arctic, which is right on the threshold of more ozone destruction, is whether will there be more or less ozone destruction in the years ahead," Dr Waters said.

"Over the next decade there's going to be a very interesting race," he said.

Dr Schoeberl said that early Aura results detected high levels of tropospheric ozone over the tropics, but without the elevated levels of carbon monoxide usually associated with biomass burning, an otherwise likely source.

"So where is this ozone coming from? It's kind of mysterious right now what's causing these high levels of ozone," Dr Schoeberl commented.

Global maps

Scientists say they are pleased with Aura's performance, despite the earlier-reported partial obstruction of the aperture on the High Resolution Dynamics Limb Sounder (HIRDLS), by a piece of plastic, apparently dislodged during take-off. Scientists said they expected to clear the lens.

All four highly sensitive instruments monitor ozone, but each also tracks specific chemicals according to their wavelengths, or frequencies.

Map of ozone hole (Nasa)
Aura is giving us new information on the Antarctic ozone hole
"Just as you can tune your radio to classical music or rock," said Dr Waters, "we build these fancy radio receivers and we tune into certain molecules."

For example, the Microwave Limb Sounder (MLS) uses microwaves to track the safe and ozone-destroying compounds of chlorine.

The Tropospheric Emission Spectrometer (TES) scans molecules that are picked up in the infrared, such as nitrogen dioxide, which is belched out from cars and trucks, and is a precursor to ozone in the lower atmosphere.

The Ozone Monitoring Instrument (OMI) detects chemicals in the visible and ultraviolet parts of the spectrum, while HIRDLS monitors the infrared.

With four instruments sweeping the sky in different bands of the electromagnetic spectrum, "it's hard for an air parcel to escape detection", said Dr Schoeberl.

In the 90 minutes or so it takes Aura to complete its orbit, the satellite can determine the abundance of key molecules in the atmosphere and compile a global map within a day.

Map of NO2 over California (Nasa)
Molecules such as nitrogen dioxide can be mapped and tracked
"All these instruments work together like instruments in a symphony to tell us more about the whole picture," said Dr Waters.

The global picture is an important one. Pollution rarely respects national boundaries. Atmospheric currents will take an aerosol particle, or molecule of gas, whip it into the stratosphere, and send it circumnavigating the globe.

"You emit a molecule of CO2 when you exhale," said Dr DeCola, "and in two weeks someone can be breathing that molecule of CO2 in Beijing, when you exhaled it in New York City."

Scientists believe Aura could revolutionize air quality monitoring in much the same way weather satellites transformed weather prediction in the 1960s and 1970s.

"We predict weather, but we can't change the weather," he said. "But we can, hopefully, predict air quality and also change air quality."

Nasa launches its Aura satellite
15 Jul 04 |  Science/Nature
Ozone benefits from treaty
01 Aug 03 |  Science/Nature
Ice mission delights scientists
14 Dec 03 |  Science/Nature
Soot 'makes global warming worse'
23 Dec 03 |  Science/Nature

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